Heretofore, a polyimide has been widely used as a protective material, an insulating material or a liquid crystal alignment film for a liquid crystal display device in the electric or electronic field by virtue of its characteristics such as high mechanical strength, heat resistance and solvent resistance. Especially in an application to the liquid crystal alignment film, a polyimide or a polyamic acid as a polyimide precursor has mostly been employed because of the uniformity and durability of the coating film surface. However, in recent years, developments in the electric and electronic fields have been remarkable, and higher properties have been correspondingly required for the material to be employed. Also for a polyimide, it has been required to impart new properties which conventional polyimides do not have.
When new properties are to be imparted to the polyimide, it is simple and effective to employ a method of introducing a new structure into a tetracarboxylic acid derivative or a diamine as a material for the polyimide and using it in combination with various materials. Especially, with the diamine as compared with the tetracarboxylic acid derivative, a compound having the desired structure introduced can easily be synthesized. Therefore, for the purpose of imparting new properties, it has been common to use a diamine having a specific structure as the material for the polyimide.
Whereas, one of the properties required for a liquid crystal alignment film is to impart a high pretilt angle to liquid crystal. In this respect, it is known that a high pretilt angle can be obtained by a polyimide liquid crystal alignment film using, as a material, a diamine having a long chain alkyl group or a fluoroalkyl group as a side chain (see, for example, JP-A-2-282726). Further, it is known that a high pretilt angle is likewise obtainable also when a diamine having an aromatic group, an aliphatic ring group or a heterocyclic ring group as a side chain is used as a material (see, for example, JP-A-3-179323).
However, along with the progress in high densification and high performance of liquid crystal displays, the stability of the pretilt angle in a process for producing a liquid crystal display device and the stability of the pretilt angle in an environment for use of the liquid crystal display device become increasingly important in addition to obtaining the high pretilt angle. For example, there was a case such that the pretilt angle decreases when a liquid crystal display device is heated at a temperature higher than the isotropic temperature of liquid crystal (hereinafter referred to as isotropic treatment). Especially, the decrease in the pretilt angle becomes particularly remarkable in a case where the pretilt angle is high or in a case where the curing temperature at the time of forming the liquid crystal alignment film, is low. Further, in a case where the curing temperature is high at the time of forming the liquid crystal alignment film, there has been a problem such that the pretilt angle can hardly be made high or tends to fluctuate. So as to solve such problems, the applicants of the present invention have already reported a polyimide liquid crystal alignment film using, as a material, a diamine having in its side chain moiety a structure comprising a cyclic substituent selected from aromatic rings, aliphatic rings and heterocyclic rings, an aliphatic ring and a long chain alkyl group (see JP-A-9-278724).
Further, as a question relating to the pretilt angle of liquid crystal, the stability of the pretilt angle in rubbing treatment may be mentioned. Especially, there will be a problem in a stable production of liquid crystal display devices if the dependence of the degree of the pretilt angle on rubbing pressure is high. However, a diamine for increasing the pretilt angle, which has been proposed heretofore, tends to be insufficient in properties by itself, and therefore it was required to suitably select a material for a polyimide to be used together with the diamine. Namely, it has been desired to produce a diamine for increasing a pretilt angle, which has not only thermal stability of the pretilt angle but also stability under the conditions of rubbing treatment.